A number of biological processes depend critically upon electron transfer: oxidative phosphorylation, photosynthesis and the redox reactions of metalloenzymes to name just a few. If a basic understanding of such systems is to be achieved, their remarkable efficiency and selectivity must be explained. The distance of separation between redox sites and the driving force of the electron transfer reaction are thought to play crucial roles in controlling the rate of electron transfer. This proposal presents an extensive program of research designed to probe the effects of distance and driving force on the rates of thermal electron transfer between metals. To that end, a series of binuclear metal complexes will be prepared where the bridging ligands are saturated structures that rigidly hold the two metals a defined distance apart and where that distance can be systematically varied. In addition, the complexes have been designed so that the difference in reduction potentials of the two metals is known and can also be changed. Two general approaches have been presented. (A) In the first system, a series of Ru2 and Os2 bipyridyl complexes will be used to indirectly probe the mechanism of electron transfer by studying the physical properties (intervalence charge transfer transitions, comproportionation constant, and electronic delocalization and coupling) of the mixed valence (M---M+) states. (B) In the second approach, a series of Ru---Os bipyridyl complexes will be employed to directly measure rates of electron transfer between the metals so as to probe the effect of distance on the rate at a fixed potential. In addition, by changing the substituents on the bipyridyl ligands bound to each metal, the reduction potentials of the two metal sites will be varied in order to study (1) the effect of redox asymmetry on the properties of the mixed valence states and (2) to determine the rate of electron transfer as a function of driving force at constant distance. There have been numerous theoretical treatments of the effects of distance and driving force on the physical properties of mixed valence complexes and on the rate of electron transfer. The data obtained through this overall approach will provide several stringent experimental tests of these theories. It is only through the systematic investigation of structurally defined binuclear metal complexes bridged by non- conjugated ligands that an accurate picture will emerge of the role played by distance and driving force in determining the properties of mixed valence states and the rate of electron transfer between metals.